Is his sister seeing something I'm not?

Last night my SO had texted his sister "goodbye" before going to bed, she texted him back "please don't do that". Now he didn't put it together that she may have been hinting at a suicidal reference.

He's been extremely stressed out at work this week, and it will continue to be stressful for him till after Saturday. He does have a history of having suicidal thoughts before I met him.

But I also believe that if she really thought he was, then she would have blown up his phone. Maybe I'm just over thinking this.